This invention relates to a heterodyne lateral grating interferometric encoder, and more particularly to such an encoder with improved modulation. Interferometers of this type are used in applications requiring relative angular measurements with submicroradian resolution and repeatability.
There are a number of problems associated with such interferometers that the presents invention addresses. A heterodyne lateral grating interferometric encoder requires the generation of an optical beam containing a sweeping sinusoidal fringe field. A Mach-Zender interferometer with acousto-optic modulators (referred to as Bragg cells or AOMs) is typically used for this purpose. Such interferometers are comprised of multiple mirrors, AOMs, and beam splitters, and alignment of all these components is difficult.
Moreover, each AOM produces a frequency shift on the first order diffracted output beam for use in the interferometer. However, much optical power can be lost in the unused orders. In addition, the use of binary or amplitude modulated gratings results in further inefficiency. As a result, the use of a higher power laser for the input to the interferometer is required.
In addition, a reference signal is required for the processing electronics that calculate the lateral position of the test grating. Additional optical components, a second reference grating and another photodetector are required to generate this electronic reference signal.